1. Field of the Invention
The present invention relates to a control system for an AC electric motor, and more particularly to a control system in which a DC link voltage of an inverter that drives an AC electric motor is variably controlled by a converter.
2. Description of the Background Art
In order to control an AC electric motor through use of a DC power source, a control system by means of an inverter is used. In particular, a structure that variably controls a DC link voltage of an inverter by a boost converter is known, as described in Japanese Patent Laying-Open No. 2004-112904 (PTD 1). As for such a control system, PTD 1 describes control for reducing the influence of dead time of a switching element (transistor) on/off of which is controlled by a boost converter to stably control a DC voltage.
Specifically, PTD 1 describes a technique for reducing the influence of dead time by making the frequency of a carrier (carrier frequency) that controls the on-off duty of the switching element lower than usual when a reactor current of the boost converter, that is, an output current from a battery, is small.
As regarded as a problem in PTD 1 as well, in such a control system that variably controls a DC link voltage of an inverter by a boost converter, fluctuations in DC voltage may lead to torque fluctuations in an AC electric motor. Particularly, in rectangular wave voltage control in which a rectangular wave voltage of a positive pulse and a negative pulse is output from an inverter in order to increase a fundamental wave component of an AC voltage applied to an AC electric motor with respect to an identical DC voltage, the above-described fluctuations in DC voltage are more likely to result in torque fluctuations than in pulse width modulation (PWM) control.
However, as described in PTD 1, if fluctuations in DC voltage are restrained by reducing the carrier frequency of the boost converter, a new problem that electromagnetic noise occurs may arise. In particular, in a control system for an AC electric motor mounted on an electric-powered vehicle, such as a hybrid vehicle, reduction in quietness in the vehicle due to the occurrence of electromagnetic noise is of concern.